The announcement came as hundreds of people gathered across the capital -- responding to a call by activists for a "procession for victims of torture and arrests" -- in some of the biggest anti-government rallies in weeks.

The teacher, Ahmed Kheir, died last Friday after being arrested during protests in the eastern town of Khashm al-Qirbam, his family said. The protest was one of the dozens against economic hardships and President Omar al-Bashir's rule, that have rattled the country since Dec. 19.

"The cause of death is from various injuries to his body from a solid, flexible object, to his back, kidneys and thighs, and between his legs," the head of an investigation committee for the public prosecutor, Amer Mohamed Ibrahim, told reporters.

Initially, a regional police chief had said that the teacher died of food poisoning. The medical report released Thursday showed no toxins were found in the teacher's body, Ibrahim said.

At least 45 people have been killed in the protests, rights groups say. The government has put the death toll at 30, including two members of security forces.

Authorities have blamed the unrest on "infiltrators" and foreign agents. "Parties hostile to Sudan" had financed a media campaign accompanying the protests, the presidency said in a statement Thursday.

In the al-Daim neighborhood in central Khartoum Thursday, about 2.5 miles south of the presidential palace, hundreds of demonstrators blocked roads with burning tires and tree branches, witnesses said.

Clashes erupted when security forces fired tear gas at the demonstrators, some of whom threw stones back at the police, according to witnesses. There were no immediate reports of casualties, but security forces were seen arresting dozens of protesters and putting them into pick-up trucks.

Security forces also used teargas against a separate, peaceful hundreds-strong march in the downtown area about 1km from the presidential palace, witnesses said.

"Peaceful, peaceful against the thieves," and "Down, that's it," they chanted - anti-government slogans that have been repeated throughout the protests.

Protests were reported in two other districts of Khartoum, including the flashpoint neighborhood of Burri. "We are determined to continue the protests till the end," a 38-year-old demonstrator who declined to be named told Reuters in central Khartoum. "We are certain the regime will fall."